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Sendmail, 3rd Edition by Bryan Costales

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How DNSBL Works

The acronym DNSBL stands for "Domain Name Services BlackList," where the term blacklist refers to the desire to prohibit all spam.

When sendmail accepts a connection from another site, one of the first things it does is get the IP address of that site. Once armed with that address, it can do a lookup of that address at a DNSBL site. To illustrate, we will use the mail-abuse.org site.[11] To see if the connecting site is an open relay site, sendmail first reverses the IP address. For example, the address 123.45.67.89 becomes 89.67.45.123. Then sendmail prefixes the hostname relays.mail-abuse.org with that reversed IP number and looks up the result as though it is a hostname:

89.67.45.123.relays.mail-abuse.org

If that hostname is found, that means the site is listed with mail-abuse.org as an open relay site. If that hostname is not found, the site is a good one.

Prior to V8.12, the rbl feature allowed you to use this DNSBL process. Beginning with V8.10, a new feature was added called dnsbl. As of V8.12, the rbl feature was removed. The enhdnsbl feature, an extended version of dnsbl, became available. These features are summarized in Table 7-1 and explained below.

Table 7-1. DNSBL features

Feature

Description

rbl

Deprecated, see dnsbl

dnsbl

Reject mail from hosts in a DNS-based rejection list

enhdnsbl

An enhanced version of dnsbl

FEATURE(dnsbl)

The dnsbl feature is used to enable the blocking of email from open relay sites, dialup sites, or known spamming sites. ...

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